The invention relates to a filling level detector for cryogenic liquids which are located in a container, which filling level detector comprises a probe, which is immersed in the liquid, and an electronics part, with a superconducting line section and a heating apparatus extending over the length of the probe, and with the voltage drop in the line section being measured as a measure of the liquid level.
The container is intended to be not only, but in particular, a container which is carried by a motor vehicle for liquefied low-boiling-point gases, for example hydrogen, which is used as fuel. Liquefied low-boiling-point gases are stored in conditions (pressure and temperature) in which the state variables of the liquid phase and of the gaseous phase differ only slightly. This makes it harder to carry out an accurate measurement using conventional methods.
However, accurate determination of the amount of fuel which is still present is necessary for use in a motor vehicle, for safety reasons and for full utilization of the range of a container filling. It is also desirable to monitor the temperature of the container contents. In addition, there are certain exacerbating factors which are typical of driving operation; for example the fact that the position of the liquid level is influenced by the inclination of the vehicle and by centrifugal forces.
A filling level detector of this generic type is known from WO 91/08449. There, the probe is a stiff rod which comprises a tubular mechanical support, a superconducting layer over the entire area, which is separated therefrom by a substrate, and a resistance heating element which is arranged in the interior of the tube and is supported by an electrical insulator on the inner wall of the tube. The heating element ensures that the suitably selected superconducting layer is not actually superconducting in the area of the cold liquid vapors, although it is in the area that is immersed in the liquid. Electrical connections for the resistance measurement are, so to speak, passed from the outside to the superconducting layer. This probe has the disadvantage of considerable physical complexity and the exposed position of the electrical connections and conductors.
The object of the invention is to provide a filling level detector which is insensitive and costs little, is suitable for mass production and can be used for containers of different sizes and different shapes.